So far Ive lost every one I put in my tank which is 3 groups of 5.They acclimate well and start eating and swimming great. Then the next day one or two will look listless and have a strange white patch (not a outer skin covering)but as if their body has been bleached on their dorsal spine.Also notice a lump of white on the lower lip of the mouth.Then they lose balance rolling upside down and finally die.My other tetras seem to be thriving great thru all this. Strange but I only have 4 rummy nose ,3 pristella,2 ottos and one cory cat that seem immune to all this trouble ,yet when I add one more rummy nose for a group of 5 several days later the 5th one dies.Water quality i excellent,Ph is 6.7in the day and 7.5 at night when the CO2 is off,kh is 4 ,NH3 is zero No2 is zero and NO3 is almost zero,CO2 is 20ppm.:fire:

That's a pretty big swing in pH. You know it is a log scale, 7.0 to 6.0 is ten times the acidity. I've read that you should try to limit pH swings to 0.2 when possible. Have you tried leaving the CO2 on overnight?

Hi Anonapersona,do you think thats a large swing from 6.7-7.5 .The books say to have the CO2 turned off at night or the Ph will go further down,and the plants dont really use it so its being wasted. But no I havent tried leaving it on during the night. I know you are using the yeast plantgro system and probably leaving it on all night.What Ph swing are you seeing? (still doesnt explain why the other fish seem effected)

That is definitely a very large pH swing. pH is a logramithic measure so 1 full point on the scale is likened to 100X as great as a tenth of a point. The other fish just might be better equipped to deal with large pH swings but the Harlequins are apparently not.

They are usually sensitive to ammonia and nitrite, but fairly tolerant of other potential water quality issues. Here's a good little article with feedback on Harlequins:Harlequin Rasboras

Also, have you purchased all your deceased Harlequins from the same source? If so - try buying them elsewhere, just to see if the store you're using has poor stock.

from a differant store along with 2 glo light tetras and 6 small neon tetras. Its been 2 days and nites and both harlequins are dead, one the first day and the second the second day. Yet the glolights and the neons are all fine!!!The one thing I notice is they seem to get a white bump on the lower lip and /or the bleached white area on the back by the dorsal fin.They get listless and start losing balance.Now they couldnt be any more sensitive than the small neons .Im really baffled.My Ph is a steady 6.7 during the day with CO2 and then when I check in the morning before the gas comes on it rises to 7.4 .Thats a .7 ph swing but it does take many hours to do this.If you buy from a Lfs, their tank could be 7.4 and you acclimate them to your tank say its 6.8 in probably an hour ,so whats the diff??

I'm sorry I can't help you with the cause of your immediate problem. I hope you can solve it soon. Keep in mind when things normalize that almost all tetras are schooling fish. They should be kept in schools of at least 8 to display their best. Their color and behavior is negatively affected when kept in smaller numbers. You might consider one nice school instead of a couple of each kind.

PH changes of more than 0.2 are stressful to the fish. We are always told to be careful when adding new fish to the tank to not subject them to big changes, heck, I don't know if it's true, it's just what I've read time and time again.

I think that the additional lowering of pH if the CO2 was left on should be investigated instead of taking the book's word on that. The book may not realize how much pH was lowered by adding the CO2. How much more the pH falls overnight may not be as significant as the rise you are subjecting them to now.

Wasting gas is insignificant when compared to wasting fish.

It would be easy enough to back off on the CO2, to maybe 10 or 15 ppm and do the testing to see what the actual day to night change is with the gas on. Then if it is reasonable, not rising overnight to lethal levels, then crank back up to the 20 ppm you are running now if you feel that you need that much. You need a balance.

15 gallons and 65 watts, that's a lot of light. You are a braver soul than I to run that sort of horsepower, I'm just too new to the game to do that yet.

As for my tanks, I have DIY CO2 on 2 tanks, the 20G and the 29G, currently testing the gelatine recipes, and Hagen on the 10G. The tap pH is 8.2, the tanks all run at 7.6 pH in the morning before lights on, 7.6 in the late afternoon -- I think the power filters blow off any excess CO2. Baby fish now in the 29G so somebody is happy.

Most fish can handle a 0.5 pH swing with no problem. It varies more than that during the day night cycle in the places most of our fish come from. A heavy rainstorm can cause a 1 point change in small streams in minutes. I run CO2 on three tanks. One DIY (10 gallon) and two pressurized (29 and 55 gallon) and I leave it on 24 hours a day. I get at most a 0.4 point swing in my pH during a 24 hour cycle. That's much less than you are getting by turning it off. I doubt though that the pH swing is what is killing your fish.

If the fish don't die you've got a big red flashing sign that says "pH Swing is the problem."

You can turn the CO2 back on at that point and let it swing like you have been doing. Exactly the same, no changes or you spoil the experiment.

One of two things will happen... The fish will then die (which will turn on the siren over the red flashing sign) -or- they will have gotten over the initial shock of being transplanted and may be healthy enough to stand the swings and live.

I appreciate all your helpful time and thought in this weird problem.The 6 neons and 2 glolites are still doing fine.Ive further added Cardinal tetras.What Rex is saying about in nature the natural ph swing is true,thats why I still dont believe its the ph swing.But I will try Lanstars test shutting off the CO2 .First though I have to buy a few more Harlequins(yikes).Sounds like I should just leave the darn gas on 24 hours if Rex is only seeing a .4 ph swing.Somehow I still feel a bit betrayed reading Amanos book clearly stating to shutoff the gas for the nite.And he wasnt the only source I read that said that!

This might sound off the wall, but if it is not a pH swing, you might have some sort of parasitic or fungal disease (the white patches you describe?) that only affects the Harlequins. The South American fish (even the relatively delicate Cardinals and Neons) might not be affected.
Maybe try treating with some broadband antibiotics just before you add the new fish and see if they survive the rigors of acclimation this time.

As well as the rummynose ,pristellas and glolites.Hmmmm just the Harlequins cant seem to cope. I doubt very much if Amano has better water or lab testing equipement than me.I work in a state of the art lab with equipment that tests down to parts per billion .(Ive been an Analytical Chemist for 22years) I should be getting some digital pics of this little heavily planted tank tonite and I will try and post it. I havent cleaned the front glass of algae in several weeks,and the Glossostigma is running all over the place along with Pennywort verticillata.Im very pleased with this hobby despite the heavy fish loss.

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